RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Apr. 16, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net loss and loss expenses			$ 2,252,424	$ 2,323,028
Reinsurance recoverable for unpaid claims and Claims adjustments	$ 3,705,793	$ 3,217,787	3,705,793	3,217,787	$ 3,501,669	$ 108,000	$ 3,159,514
Net (favorable) adverse prior year reserve development	(26,727)	(45,660)	(65,021)	(160,083)
Short Tail Reserving Class Insurance and Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(2,000)	(12,000)	50,000	(92,000)
Liability Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development		(11,000)	(26,000)	(19,000)
Motor Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(23,000)	(7,000)	(34,000)	(15,000)
Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(14,609)	(13,478)	(42,849)	(60,547)
Insurance | Short Tail Reserving Class Insurance and Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(11,000)		4,000
Insurance | Marine and Aviation | Short Tail Reserving Class Insurance and Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(3,000)
Insurance | Marine | Short Tail Reserving Class Insurance and Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development			(24,000)
Insurance | Professional Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(4,000)	(10,000)	(14,000)	(12,000)
Insurance | Credit and Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development			(10,000)
Insurance | Liability Insurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	4,000	11,000		18,000
Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(12,118)	(32,182)	(22,172)	(99,536)
Reinsurance | Short Tail Reserving Class Insurance and Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	12,000		71,000
Reinsurance | Credit and Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	(6,000)	(6,000)	(33,000)	(21,000)
Reinsurance | Professional Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year reserve development	$ 7,000	$ (10,000)		(18,000)
Syndicate 2007
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Policyholder benefits and claims incurred, assumed and ceded				819,000
Catastrophe and Weather-related Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net loss and loss expenses			$ 206,000	$ 162,000